Photonic crystals are periodic, dielectric, composite structures in which the interfaces between the dielectric media behave as scattering centres for light. Photonic crystals consist of at least two component materials (one of which may be air) having different refractive indices. The materials are arranged alternatingly in a periodic manner that is scaled so as to interfere with the propagation of light in a particular wavelength range. Light is scattered at the interfaces between the materials due to differences in the refractive index (or refractive index contrast) of the two materials. The periodic arrangement of the scattering interfaces prevents light with wavelengths comparable to the periodicity dimension of the photonic crystal from propagating through the structure. The band of blocked or forbidden wavelengths is commonly referred to as a photonic bandgap.
Practical applications for photonic crystals generally require man-made structures. Photonic devices are designed primarily for light frequencies ranging from the ultraviolet to the microwave regime (corresponding to wavelengths from 10 nanometers to 10 centimeters, respectively). Photonic crystals having these corresponding periodicities are not readily available in nature.
The simplest photonic crystal structure is a multilayer stack, consisting of alternating layers of dielectric materials with different refractive indices. The period of the structure (or unit cell dimension) is the combined thickness of a single layer of each of the dielectric materials. Such structures offer periodic refractive index contrast in one direction only and are known as one dimensional (1D) photonic crystals.
A 1D photonic crystal theoretically can act as a perfect mirror (i.e., having 100% reflectivity) for light with wavelengths within its photonic bandgap, and incident normal to the multilayer surface. Such 1D photonic crystals can be used in a variety of optical devices, including dielectric mirrors and optical filters.
When the periodic refractive index contrast is extended to two (or three) directions, the structures are known as 2D (or 3D) photonic crystals. In 2D photonic crystals, light may be reflected from any angle incident in the plane of periodicity (within its photonic bandgap), while for 3D photonic crystals, light may be reflected from any angle of incidence (within its photonic bandgap) 3D photonic crystal structures exhibiting this property have full photonic bandgaps.
Practical applications of photonic crystals generally depend on intentionally introducing defects into the periodic structure so that the propagation and/or confinement of light with wavelengths that would otherwise be forbidden can occur, that is, through so-called defect states located within the photonic bandgap.
Defects are defined as regions of the photonic crystal having a different geometry (i.e., spacing and/or symmetry) and/or a different refractive index contrast from that of the general periodic structure. For example, in a photonic crystal comprised of a periodic array of air-holes within a dielectric sheet, a possible defect would include leaving an array position with the dielectric material intact and not having an air-hole at that location. Given appropriate characteristics of the dielectric sheet and lattice symmetry, an optical cavity in the vicinity of the defect can form, suitable for confining at least one mode of light.
As another example, in the case of an array of dielectric columns separated by airspaces, removal of a series of columns (in a line), would create a defect through which specific wavelengths of light otherwise forbidden would be able to propagate. By appropriately eliminating further columns, light may be directed to form all-optical circuits (i.e., so-called planar lightwave circuits). Such circuits benefit from extremely tight bend radii to furnish compact optical circuits.
It has been recognized that photonic crystals may also provide imperfect reflectivity (i.e., reflectivity less than unity) due to imperfections in the periodic structure, Such imperfections can act in a manner similar to defect states, but occur through inadvertence and often result from limitations in the fabrication process. Although fabrication techniques have improved, unintended imperfections continue to occur and no practical means has been proposed to remove or correct them.
Defect states in 1D, 2D, and 3D photonic crystals result in planar, linear, and point localization, respectively. Since the presence of defect states in photonic crystal structures can precisely control light propagation or confinement, the design of photonic crystal-based optical devices has been extensively explored. Practical applications of photonic crystals to date include waveguides, light cavities, high a filters, channel drop filters and mirrors.
There has recently also been great interest in exploring the use of photonic crystals for applications such as planar lightwave circuits, wavelength division multiplexing applications, optical switches, optical computing, tunable gates, interconnects, and so forth. In such applications, the defect state in the photonic crystal would have to be altered in a controlled fashion to create a tunable wavelength band that can propagate through or be confined in the device. The main limitation of traditional photonic crystals is that control over the propagation or confinement of light is determined and fixed by its physical structure, as the defect state is permanently fixed in the photonic crystal, Although fixed defects in photonic crystals offer an ability to control light propagation or confinement, once such defects are introduced, the propagation or confinement of light in the crystal is determined. Thus, discretionary switching of light, for example, or re-routing of optical signals, is not available with fixed defects in a photonic crystal.
In order to configure the propagation or confinement of light through a photons crystal, the main approach that has been investigated is the modulation of refractive index contrast, principally using an applied electric field, and the uniform adjustment of the arrangements of the dielectric elements in the whole photonic crystal.
For example, U.S. Pat. No. 6,058,127 (Joannopoulos et al) discloses a technique for refractive index contrast modulation by applying an electric field to the dielectric. However, known materials offer only low electro-optic coefficient values for applied electric fields below the breakdown limit, and thus yield only small changes in refractive index.
In U.S. Pat. No. 5,973,823 (Koops et al.) and U.S. Pat. No. 6,064,506 (Koops), the photonic crystal cavities were filled with a material adjustable by an electric field. In this case, the refractive index contrast between the two media is significantly lowered, limiting the possibility of obtaining a full photonic bandgap.
An example of adjusting the photonic crystal periodicity is through the use of temperature, pressure or field excitation to the photonic crystal as described in U.S. Pat. No. 5,688,318 (Milstein et al.), but the proposed method does not provide discretionary modification of selected elements of the photonic device. No control of the defect states is provided.
None of the previously disclosed approaches for tuning photonic crystals has been successful in practical applications because they either provide only negligible changes in the ability of the structures to guide or confine light, or they significantly reduce the size of the photonic band gap, or they produce only non-local changes that are not useful for re-routing or confining light in a discretionary fashion.
It is an object of the present invention therefore to obviate or mitigate the shortcomings of known photonic crystal structures, and particularly to provide photonic crystal structures through which it is possible to change the propagation and/or confinement of light in a discretionary fashion,